Small transversely-excited film bulk acoustic resonators with enhanced q-factor

ABSTRACT

An acoustic resonator device includes a conductor pattern formed on a surface of a piezoelectric plate. The conductor pattern includes a first busbar, a second busbar, and n interleaved parallel fingers of an interdigital transducer (IDT), where n is a positive integer. The fingers extend alternately from the first and second busbars. A first finger and an n&#39;th finger are disposed at opposing ends of the IDT. The conductor pattern also includes a first reflector element proximate and parallel to the first finger and a second reflector element proximate and parallel to the n&#39;th finger. When an RF signal is applied between the first and second busbars, the first reflector element is at substantially the same potential as the first finger and the second reflector element is at substantially the same potential as the n&#39;th finger.

RELATED APPLICATION INFORMATION

This patent claim priority to the following provisional patentapplications: application No. 63/012,849, filed Apr. 20, 2020, entitledSMALL HIGH Q XBAR RESONATORS; application No. 63/066,520, filed Aug. 17,2020, entitled SMALL FEFLECTOTS TO IMPROVE XBAR LOSS; and applicationNo. 63/074,991, filed Sep. 4, 2020, entitled SMALL REFLECTORS TO IMPROVEPERFORMANCE OF TRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATORS AT ASPECIFIED FREQUENCY. All of these applications are incorporated hereinby reference.

NOTICE OF COPYRIGHTS AND TRADE DRESS

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. This patent document may showand/or describe matter which is or may become trade dress of the owner.The copyright and trade dress owner has no objection to the facsimilereproduction by anyone of the patent disclosure as it appears in thePatent and Trademark Office patent files or records, but otherwisereserves all copyright and trade dress rights whatsoever.

BACKGROUND Field

This disclosure relates to radio frequency filters using acoustic waveresonators, and specifically to filters for use in communicationsequipment.

Description of the Related Art

A radio frequency (RF) filter is a two-port device configured to passsome frequencies and to stop other frequencies, where “pass” meanstransmit with relatively low signal loss and “stop” means block orsubstantially attenuate. The range of frequencies passed by a filter isreferred to as the “pass-band” of the filter. The range of frequenciesstopped by such a filter is referred to as the “stop-band” of thefilter. A typical RF filter has at least one pass-band and at least onestop-band. Specific requirements on a pass-band or stop-band depend onthe application. For example, a “pass-band” may be defined as afrequency range where the insertion loss of a filter is better than adefined value such as 1 dB, 2 dB, or 3 dB. A “stop-band” may be definedas a frequency range where the rejection of a filter is greater than adefined value such as 20 dB, 30 dB, 40 dB, or greater depending onapplication.

RF filters are used in communications systems where information istransmitted over wireless links. For example, RF filters may be found inthe RF front-ends of cellular base stations, mobile telephone andcomputing devices, satellite transceivers and ground stations, IoT(Internet of Things) devices, laptop computers and tablets, fixed pointradio links, and other communications systems. RF filters are also usedin radar and electronic and information warfare systems.

RF filters typically require many design trade-offs to achieve, for eachspecific application, the best compromise between performance parameterssuch as insertion loss, rejection, isolation, power handling, linearity,size and cost. Specific design and manufacturing methods andenhancements can benefit simultaneously one or several of theserequirements.

Performance enhancements to the RF filters in a wireless system can havebroad impact to system performance. Improvements in RF filters can beleveraged to provide system performance improvements such as larger cellsize, longer battery life, higher data rates, greater network capacity,lower cost, enhanced security, higher reliability, etc. Theseimprovements can be realized at many levels of the wireless system bothseparately and in combination, for example at the RF module, RFtransceiver, mobile or fixed sub-system, or network levels.

High performance RF filters for present communication systems commonlyincorporate acoustic wave resonators including surface acoustic wave(SAW) resonators, bulk acoustic wave (BAW) resonators, film bulkacoustic wave resonators (FBAR), and other types of acoustic resonators.However, these existing technologies are not well-suited for use at thehigher frequencies and bandwidths proposed for future communicationsnetworks.

The desire for wider communication channel bandwidths will inevitablylead to the use of higher frequency communications bands. Radio accesstechnology for mobile telephone networks has been standardized by the3GPP (3^(rd) Generation Partnership Project). Radio access technologyfor 5^(th) generation (5G) mobile networks is defined in the 5G NR (newradio) standard. The 5G NR standard defines several new communicationsbands. Two of these new communications bands are n77, which uses thefrequency range from 3300 MHz to 4200 MHz, and n79, which uses thefrequency range from 4400 MHz to 5000 MHz. Both band n77 and band n79use time-division duplexing (TDD), such that a communications deviceoperating in band n77 and/or band n79 use the same frequencies for bothuplink and downlink transmissions. Bandpass filters for bands n77 andn79 must be capable of handling the transmit power of the communicationsdevice. WiFi bands at 5 GHz and 6 GHz also require high frequency andwide bandwidth. The 5G NR standard also defines millimeter wavecommunication bands with frequencies between 24.25 GHz and 40 GHz.

The Transversely-Excited Film Bulk Acoustic Resonator (XBAR) is anacoustic resonator structure for use in microwave filters. The XBAR isdescribed in U.S. Pat. No. 10,491,291, titled TRANSVERSELY EXCITED FILMBULK ACOUSTIC RESONATOR. An XBAR resonator comprises an interdigitaltransducer (IDT) formed on a thin floating layer, or diaphragm, of asingle-crystal piezoelectric material. The IDT includes a first set ofparallel fingers, extending from a first busbar and a second set ofparallel fingers extending from a second busbar. The first and secondsets of parallel fingers are interleaved. A microwave signal applied tothe IDT excites a shear primary acoustic wave in the piezoelectricdiaphragm. XBAR resonators provide very high electromechanical couplingand high frequency capability. XBAR resonators may be used in a varietyof RF filters including band-reject filters, band-pass filters,duplexers, and multiplexers. XBARs are well suited for use in filtersfor communications bands with frequencies above 3 GHz.

DESCRIPTION OF THE DRAWINGS

FIG. 1 includes a schematic plan view, two schematic cross-sectionalviews, and a detail view of a transversely-excited film bulk acousticresonator (XBAR).

FIG. 2 is a schematic block diagram of a band-pass filter using acousticresonators.

FIG. 3 is a graph of relationships between the Q-factor of an XBAR andthe number of fingers in the interdigital transducer (IDT) of the XBAR.

FIG. 4 is a schematic plan view of an IDT with reflector elements.

FIG. 5 is a schematic plan view of another IDT with reflector elements.

FIG. 6 is a graph comparing the normalized Q-factor of an XBAR with andwithout reflector elements at the resonance frequency.

FIG. 7 is a graph comparing the normalized Q-factor of an XBAR with andwithout reflector elements at the anti-resonance frequency.

FIG. 8 is a graph showing relative Q-factor as a function of reflectorelement pitch and mark for a representative XBAR at a frequency of 5150MHz.

FIG. 9 is a graph showing relative Q-factor as a function of frequencyand reflector element mark for an XBAR with two reflector elements ateach end.

FIG. 10 is a graph showing relative Q-factor as a function of frequencyand reflector element mark for an XBAR with one reflector element ateach end.

FIG. 11 is a graph showing relative Q-factor as a function of frequencyand reflector element mark for an XBAR with five reflector elements ateach end.

FIG. 12 is a graph comparing the performance of two band-pass filtersusing XBARs with and without reflector elements.

FIG. 13 is a flow chart of a method for fabricating an XBAR or a filterusing XBARs.

Throughout this description, elements appearing in figures are assignedthree-digit or four-digit reference designators, where the two leastsignificant digits are specific to the element and the one or two mostsignificant digit is the figure number where the element is firstintroduced. An element that is not described in conjunction with afigure may be presumed to have the same characteristics and function asa previously-described element having the same reference designator.

DETAILED DESCRIPTION

Description of Apparatus

FIG. 1 shows a simplified schematic top view and orthogonalcross-sectional views of an XBAR 100. XBAR resonators such as theresonator 100 may be used in a variety of RF filters includingband-reject filters, band-pass filters, duplexers, and multiplexers.

The XBAR 100 is made up of a thin film conductor pattern formed on asurface of a piezoelectric plate 110 having parallel front and backsurfaces 112, 114, respectively. The piezoelectric plate is a thinsingle-crystal layer of a piezoelectric material such as lithiumniobate, lithium tantalate, lanthanum gallium silicate, gallium nitride,or aluminum nitride. The piezoelectric plate is cut such that theorientation of the X, Y, and Z crystalline axes with respect to thefront and back surfaces is known and consistent. The piezoelectric platemay be Z-cut, which is to say the Z axis is normal to the front and backsurfaces 112, 114. The piezoelectric plate may be rotated Z-cut orrotated YX-cut. XBARs may be fabricated on piezoelectric plates withother crystallographic orientations.

The back surface 114 of the piezoelectric plate 110 is attached to asurface of a substrate 120 except for a portion of the piezoelectricplate 110 that forms a diaphragm 115 spanning a cavity 140 formed in thesubstrate. The portion of the piezoelectric plate that spans the cavityis referred to herein as the “diaphragm” 115 due to its physicalresemblance to the diaphragm of a microphone. As shown in FIG. 1, thediaphragm 115 is contiguous with the rest of the piezoelectric plate 110around all of a perimeter 145 of the cavity 140. In this context,“contiguous” means “continuously connected without any interveningitem”. In other configurations, the diaphragm 115 may be contiguous withthe piezoelectric plate around at least 50% of the perimeter 145 of thecavity 140.

The substrate 120 provides mechanical support to the piezoelectric plate110. The substrate 120 may be, for example, silicon, sapphire, quartz,or some other material or combination of materials. The back surface 114of the piezoelectric plate 110 may be bonded to the substrate 120 usinga wafer bonding process. Alternatively, the piezoelectric plate 110 maybe grown on the substrate 120 or attached to the substrate in some othermanner. The piezoelectric plate 110 may be attached directly to thesubstrate or may be attached to the substrate 120 via one or moreintermediate material layers (not shown in FIG. 1).

“Cavity” has its conventional meaning of “an empty space within a solidbody.” The cavity 140 may be a hole completely through the substrate 120(as shown in Section A-A and Section B-B) or a recess in the substrate120 under the diaphragm 115. The cavity 140 may be formed, for example,by selective etching of the substrate 120 before or after thepiezoelectric plate 110 and the substrate 120 are attached.

The conductor pattern of the XBAR 100 includes an interdigitaltransducer (IDT) 130. The IDT 130 includes a first plurality of parallelfingers, such as finger 136, extending from a first busbar 132 and asecond plurality of fingers extending from a second busbar 134. The term“busbar” means a conductor from which the fingers of an IDT extend. Thefirst and second pluralities of parallel fingers are interleaved. Theinterleaved fingers overlap for a distance AP, commonly referred to asthe “aperture” of the IDT. The center-to-center distance L between theoutermost fingers of the IDT 130 is the “length” of the IDT.

The first and second busbars 132, 134 serve as the terminals of the XBAR100. A radio frequency or microwave signal applied between the twobusbars 132, 134 of the IDT 130 excites a primary acoustic mode withinthe piezoelectric plate 110. The primary acoustic mode is a bulk shearmode where acoustic energy propagates along a direction substantiallyorthogonal to the surface of the piezoelectric plate 110, which is alsonormal, or transverse, to the direction of the electric field created bythe IDT fingers. Thus, the XBAR is considered a transversely-excitedfilm bulk wave resonator.

The IDT 130 is positioned on the piezoelectric plate 110 such that atleast the fingers of the IDT 130 are disposed on the diaphragm 115 thatspans, or is suspended over, the cavity 140. As shown in FIG. 1, thecavity 140 has a rectangular shape with an extent greater than theaperture AP and length L of the IDT 130. A cavity of an XBAR may have adifferent shape, such as a regular or irregular polygon. The cavity ofan XBAR may more or fewer than four sides, which may be straight orcurved.

For ease of presentation in FIG. 1, the geometric pitch and width of theIDT fingers are greatly exaggerated with respect to the length(dimension L) and aperture (dimension AP) of the XBAR. A typical XBARhas more than ten parallel fingers in the IDT 130. An XBAR may havehundreds, possibly thousands, of parallel fingers in the IDT 130.Similarly, the thicknesses of the IDT fingers and the piezoelectricplate in the cross-sectional views are greatly exaggerated.

Referring now to the detailed schematic cross-sectional view, afront-side dielectric layer 150 may optionally be formed on the frontside of the piezoelectric plate 110. The “front side” of the XBAR is, bydefinition, the surface facing away from the substrate. The front-sidedielectric layer 150 may be formed only between the IDT fingers (e.g.IDT finger 138 b) or may be deposited as a blanket layer such that thedielectric layer is formed both between and over the IDT fingers (e.g.IDT finger 138 a). The front-side dielectric layer 150 may be anon-piezoelectric dielectric material, such as silicon dioxide, alumina,or silicon nitride. A thickness of the front side dielectric layer 150is typically less than about one-third of the thickness tp of thepiezoelectric plate 110. The front-side dielectric layer 150 may beformed of multiple layers of two or more materials. In someapplications, a back-side dielectric layer (not shown) may be formed onthe back side of the piezoelectric plate 110.

The IDT fingers 138 a, 138 b may be one or more layers of aluminum, analuminum alloy, copper, a copper alloy, beryllium, gold, tungsten,molybdenum, chromium, titanium or some other conductive material. TheIDT fingers are considered to be “substantially aluminum” if they areformed from aluminum or an alloy comprising at least 50% aluminum. TheIDT fingers are considered to be “substantially copper” if they areformed from copper or an alloy comprising at least 50% copper. Thin(relative to the total thickness of the conductors) layers of metalssuch as chromium or titanium may be formed under and/or over and/or aslayers within the fingers to improve adhesion between the fingers andthe piezoelectric plate 110 and/or to passivate or encapsulate thefingers and/or to improve power handling. The busbars (132, 134 inFIG. 1) of the IDT may be made of the same or different materials as thefingers.

Dimension p is the center-to-center spacing or “pitch” of the IDTfingers, which may be referred to as the pitch of the IDT and/or thepitch of the XBAR. Dimension m is the width or “mark” of the IDTfingers. The geometry of the IDT of an XBAR differs substantially fromthe IDTs used in surface acoustic wave (SAW) resonators. In a SAWresonator, the pitch of the IDT is one-half of the acoustic wavelengthat the resonance frequency. Additionally, the mark-to-pitch ratio of aSAW resonator IDT is typically close to 0.5 (i.e. the mark or fingerwidth is about one-fourth of the acoustic wavelength at resonance). Inan XBAR, the pitch p of the IDT may be 2 to 20 times the width w of thefingers. The pitch p is typically 3.3 to 5 times the width w of thefingers. In addition, the pitch p of the IDT may be 2 to 20 times thethickness of the piezoelectric plate 210. The pitch p of the IDT istypically 5 to 12.5 times the thickness of the piezoelectric plate 210.The width m of the IDT fingers in an XBAR is not constrained to be nearone-fourth of the acoustic wavelength at resonance. For example, thewidth of XBAR IDT fingers may be 500 nm or greater, such that the IDTcan be readily fabricated using optical lithography. The thickness ofthe IDT fingers may be from 100 nm to about equal to the width m. Thethickness of the busbars (132, 134) of the IDT may be the same as, orgreater than, the thickness tm of the IDT fingers.

FIG. 2 is a schematic circuit diagram and layout for a high frequencyband-pass filter 200 using XBARs. The filter 200 has a conventionalladder filter architecture including three series resonators 210A, 210B,210C and two shunt resonators 220A, 220B. The three series resonators210A, 210B, and 210C are connected in series between a first port and asecond port (hence the term “series resonator”). In FIG. 2, the firstand second ports are labeled “In” and “Out”, respectively. However, thefilter 200 is bidirectional and either port may serve as the input oroutput of the filter. The two shunt resonators 220A, 220B are connectedfrom nodes between the series resonators to ground. A filter may containadditional reactive components, such as capacitors and/or inductors, notshown in FIG. 2. All the shunt resonators and series resonators areXBARs. The inclusion of three series and two shunt resonators isexemplary. A filter may have more or fewer than five total resonators,more or fewer than three series resonators, and more or fewer than twoshunt resonators. Typically, all of the series resonators are connectedin series between an input and an output of the filter. All of the shuntresonators are typically connected between ground and the input, theoutput, or a node between two series resonators.

In the exemplary filter 200, the three series resonators 210A, B, C andthe two shunt resonators 220A, B of the filter 200 are formed on asingle plate 230 of piezoelectric material bonded to a silicon substrate(not visible). In some filters, the series resonators and shuntresonators may be formed on different plates of piezoelectric material.Each resonator includes a respective IDT (not shown), with at least thefingers of the IDT disposed over a cavity in the substrate. In this andsimilar contexts, the term “respective” means “relating things each toeach”, which is to say with a one-to-one correspondence. In FIG. 2, thecavities are illustrated schematically as the dashed rectangles (such asthe rectangle 235). In this example, each IDT is disposed over arespective cavity. In other filters, the IDTs of two or more resonatorsmay be disposed over a single cavity.

Each of the resonators 210A, 210B, 210C, 220A, 220B in the filter 200has resonance where the admittance of the resonator is very high and ananti-resonance where the admittance of the resonator is very low. Theresonance and anti-resonance occur at a resonance frequency and ananti-resonance frequency, respectively, which may be the same ordifferent for the various resonators in the filter 200. Inover-simplified terms, each resonator can be considered a short-circuitat its resonance frequency and an open circuit at its anti-resonancefrequency. The input-output transfer function will be near zero at theresonance frequencies of the shunt resonators and at the anti-resonancefrequencies of the series resonators. In a typical filter, the resonancefrequencies of the shunt resonators are positioned below the lower edgeof the filter's passband and the anti-resonance frequencies of theseries resonators are positioned above the upper edge of the passband.In some filters, a front-side dielectric layer (also called a “frequencysetting layer”), represented by the dot-dash rectangle 270, may beformed on the shunt resonators to set the resonance frequencies of theshunt resonators lower relative to the resonance frequencies of theseries resonators.

The Q-factor of an acoustic resonator is commonly defined as the peakenergy stored during a cycle of the applied RF signal divided by thetotal energy dissipated or lost during the cycle. The Q-factor of anXBAR is a complex function of numerous parameters including the length,or number of fingers, in the IDT of the XBAR.

Possible loss mechanisms in an acoustic resonator include resistivelosses in the IDT and other conductors; viscous or acoustic losses inthe piezoelectric plate, IDT fingers, and other materials; and leakageof acoustic energy out of the resonator structure. The peak energystored in a resonator is proportional the capacitance of the resonator.In an XBAR resonator, the capacitance is proportional to the number ofIDT fingers. Resistive losses and viscose losses are also proportionalto the number of IDT fingers. Acoustic energy that leaks from theresonator in the transverse direction (i.e. the direction parallel tothe IDT fingers) is proportional to the length of the resonator and thusalso proportional to the number of IDT fingers. In contrast, energy lostfrom the ends of the IDT in the longitudinal direction (i.e. thedirection normal to the IDT fingers) is roughly constant, independent ofthe number of IDT fingers. As the number of IDT fingers and the peakenergy stored in an XBAR is reduced, the acoustic energy lost in thelongitudinal direction becomes an ever-increasing fraction of the peakenergy stored.

FIG. 3 is a graph of the normalized Q factor of a representative XBAR asa function of the number of fingers in the IDT of the XBAR. The“normalized Q-factor” is the Q-factor of the XBAR with a finite numberof IDT fingers divided by the Q-factor of a hypothetical XBAR with thesame structure and an infinite number of IDT fingers. In FIG. 3,normalized Q-factor is quantified as a percentage of the Q-factor of theXBAR with an infinite number of IDT fingers. Specifically, the solidcurve 310 is a plot of the normalized Q-factor at the resonancefrequency and the dashed curve 320 is a plot of the normalized Q-factorat the anti-resonance frequency. The data in FIG. 3 is derived fromsimulations using a finite element method.

FIG. 3 shows that the normalized Q-factor of an XBAR with a finitenumber of IDT fingers is less than 100%, which is to say the Q-factor ofan XBAR with a finite number of IDT fingers is less than the Q-factor ofa similar XBAR with an infinite number of IDT fingers. Although notshown in FIG. 3, the normalized Q-factor of an XBAR may asymptoticallyapproach 100% for a very large number of IDT fingers. As anticipated,the normalized Q-factor depends on the number of IDT fingers. Inparticular, the normalized Q-factor decreases precipitously for XBARswith less than about 20 IDT fingers due to the increasing significanceof acoustic energy lost in the longitudinal direction.

FIG. 4 is a plan view of an exemplary conductor pattern 400 that reducesthe acoustic energy leakage in the longitudinal direction at the ends ofan XBAR. The conductor pattern 400 includes an IDT 430 and fourreflector elements 462, 464, 466, 468. The IDT 430 includes a firstbusbar 432, a second busbar 434, and a plurality of n interleaved IDTfingers extending alternately from the first and second busbars. In thisexample, n, the number of IDT fingers, is equal to 24. In other XBARs, nmay be in a range from 20 to 100 or more IDT fingers. IDT finger 436 isthe 1^(st) finger and IDT finger 438 is the n'th finger. Numbering theIDT fingers from left to right (as shown in FIG. 4) is arbitrary and thedesignations of the 1^(st) and n'th fingers could be reversed.

As shown in FIG. 4, the odd numbered IDT fingers extend from the firstbusbar 432 and the even numbered IDT fingers extend from the secondbusbar 434. The IDT 430 has an even number of IDT fingers such that the1^(st) and n'th IDT fingers 436, 438 extend from different busbars. Insome cases, an IDT may have an odd number of IDT fingers such that the1^(st) and n'th IDT fingers and all of the reflector elements extendfrom the same busbar.

A total of four reflector elements are provided outside of periphery ofthe IDT 430. A first reflector element 462 is proximate and parallel to1st IDT finger 436 at the left end of the IDT 430. A second reflectorelement 466 is proximate and parallel to n'th IDT finger 438 at theright end of the IDT 430. An optional third reflector element 464 isparallel to the first reflector element 462. An optional fourthreflector element 468 is parallel to the second reflector element 466.

First and third reflector elements 462, 464 extend from the first busbar432 and thus are at the same electrical potential as the 1st IDT finger436. Similarly, second and fourth reflector elements 466 and 468 extendfrom the second busbar 430 and thus are at the same electrical potentialas the n'th IDT finger 438.

The reflector elements 462, 464, 466, 468 are configured to confineacoustic energy to the area of the IDT 430 and thus reduce acousticenergy losses in the longitudinal direction. To this end, the pitch prbetween adjacent reflector elements and between reflector elements 462and 466 and the adjacent first and n'th IDT fingers, respectively, istypically greater than the pitch p of the IDT fingers. The width or markmr of the reflector elements 462, 464, 466, 468 is not necessarily equalto the mark m of the IDT fingers. As will be described subsequently, themark mr of the reflector elements may be selected to optimize Q-factorat a specific frequency or range of frequencies.

FIG. 5 is a plan view of another conductor pattern 500 that reduces theacoustic energy leakage in the longitudinal direction at the ends of anXBAR. The conductor pattern 500 includes an IDT 530 and four reflectorelements 562, 564, 566, 568. The IDT 530 includes a first busbar 532, asecond busbar 534, and a plurality of interleaved IDT fingers extendingalternately from the first and second busbars as previously described.IDT fingers 536 and 538 are the 1^(st) and n'th IDT fingers at the leftand right (as shown in FIG. 5) ends of the IDT 530.

A total of four reflector elements are provided outside of periphery ofthe IDT 530. First and third reflector elements 562 and 564 areproximate and parallel to 1st IDT finger 536 at the left end of the IDT530. First and third reflector elements 562, 564 are connected to eachother but are not connected to either busbar 532, 534. First and thirdreflector elements 562, 564 are capacitively coupled to 1st IDT finger536 and thus are at substantially the same electrical potential as the1st IDT finger 536. The reflector elements are considered to be atsubstantially the same potential if, when an RF signal is appliedbetween the busbars 532, 534, the potential between the reflectorelements and the 1^(st) IDT finger is small compared to the potentialbetween adjacent IDT fingers.

Similarly, second and fourth reflector elements 566 and 568 areproximate and parallel to n'th IDT finger 538 at the right end of theIDT 530. Second and fourth reflector elements 566, 568 are connected toeach other and not connected to each other or either busbar 532, 534.Second and fourth reflector elements 566, 568 are capacitively coupledto each other and to n'th IDT finger 538 and thus are at nearly the sameelectrical potential as the n'th IDT finger 538.

The reflector elements 562, 564, 566, 568 are configured to confineacoustic energy to the area of the IDT 530 and thus reduce acousticenergy lost in the longitudinal direction. To this end, the pitch prbetween adjacent reflector elements and between reflector elements 562and 566 and the adjacent terminal IDT fingers is typically greater thanthe pitch p of the IDT fingers. The width or mark mr of the reflectorelements 562, 564, 566, 568 is not necessarily equal to the mark m ofthe IDT fingers. The mark mr of the reflector elements may be selectedto optimize Q-factor for a particular frequency of range of frequencies.

FIG. 6 is a graph of the normalized Q-factor as a function of number ofIDT fingers for another XBAR with and without reflector elements similarto the reflector elements shown in FIG. 4. Specifically, the solid curve610 is a plot of the normalized Q-factor of an XBAR without reflectorelements at its resonance frequency. The dashed curve 620 is a plot ofthe normalized Q-factor at the resonance frequency for a similar XBARwith two reflector elements at each side of the IDT. In both cases, thepiezoelectric plate is lithium niobate 400 nm thick, the IDT fingers arealuminum 500 nm thick, IDT pitch p=4 microns, and IDT finger mark m=1micron. For the XBAR with reflector elements, pr=4.2 microns andmr=0.735 microns. With reflector elements, an XBAR with as few as 10fingers can have a normalized Q-factor up to 80%.

FIG. 7 is a graph of the normalized Q-factor as a function of number ofIDT fingers for another XBAR with and without reflector elements similarto the reflector elements shown in FIG. 4. Specifically, the solid curve710 is a plot of the normalized Q-factor of an XBAR without reflectorelements at its anti-resonance frequency. The dashed curve 720 is a plotof the normalized Q-factor at the anti-resonance frequency for a similarXBAR with two reflector elements at each side of the IDT. In both cases,the piezoelectric plate is lithium niobate 400 nm thick, the IDT fingersare aluminum 500 nm thick, IDT pitch p=4 microns, and IDT finger markm=1 micron. For the XBAR with reflector elements, pr=8 microns andmr=0.80 microns. With reflector elements, an XBAR with as few as 14fingers can have a normalized Q-factor up to 80%.

FIG. 8 shows a chart 800 illustrating the relationship between the pitchpr and mark mr of reflector elements of an exemplary XBAR device at afixed frequency of 5150 MHz. The exemplary XBAR device has a lithiumniobate piezoelectric plate with a thickness of 400 nm and aluminum IDTand reflector elements with a thickness of 500 nm. The pitch and mark ofthe IDT fingers are 4 microns and 1 micron, respectively. There are tworeflector elements at each end of the IDT. The lighter shaded area 810A,810B, 810C, 810D identify combinations of pr and mr where the normalizedQ-factor is greater than or equal to 85%. The darker shaded area 820A,820B, 820C, 820D identify combinations of pr and mr where the normalizedQ-factor is greater than or equal to 90%. For comparison, the normalizedQ-factor of this XBAR without reflector elements is 74% at 5150 MHz.Although not identified in FIG. 8 there are combinations of pr and mrwhere the normalized Q-factor is less than 75%, indicating improperlyconfigured reflector elements can degrade XBAR Q-factor.

There are multiple combinations of pr and mr that raise the normalizedQ-factor to 85% or 90%. To achieve a normalized Q-factor of greater thanor equal to 90%, pr must be greater than or equal to 1.2 times the pitchp of the IDT fingers. For pr=6 microns (1.5p), there are at least fourvalues of mr that raise the normalized Q-factor to more than 90%.

FIG. 9 shows a chart 900 illustrating the relationship between the markmr of reflector elements and frequency for an exemplary XBAR device withtwo reflector elements at each side of an IDT and pr=5.2 microns. As inthe previous examples, the exemplary XBAR device has a lithium niobatepiezoelectric plate with a thickness of 400 nm and aluminum IDT andreflector elements with a thickness of 500 nm. The pitch and mark of theIDT fingers are 4 microns and 1 micron, respectively. Lighter shadedareas such as area 910 identify combinations of frequency and mr wherethe normalized Q-factor is greater than or equal to 85%. Darker shadedareas such as area 920 identify combinations of frequency and mr wherethe normalized Q-factor is greater than or equal to 90%. For comparison,the normalized Q-factor of this XBAR without reflector elements is 74%at 5150 MHz.

The chart 900 illustrates that, for a particular reflector element pitchpr, reflector element mark mr must be selected with consideration of thefrequency where the Q-factor of an XBAR needs improvement. For example,selecting mr=0.95 microns provides a normalized Q-factor greater than90% over a frequency range from about 4980 MHz to greater than 5200 MHz.Selecting mr=1.7 microns provides a normalized Q-factor greater than 90%over a frequency range of less than 4700 MHz to about 4950 MHz. However,selecting mr=1.7 microns may actually lower the Q-factor at 5200 MHzcompared to an XBAR with no reflector elements.

FIG. 10 shows a chart 11000 illustrating the relationship between themark mr of reflector elements and frequency for an exemplary XBAR devicewith one reflector element at each side of an IDT and pr=5.2 microns.The exemplary XBAR device is the same as the previous examples. As inFIG. 9, lighter shaded areas such as area 1010 identify combinations offrequency and mr where the normalized Q-factor is greater than or equalto 85%. Darker shaded areas such as area 1020 identify combinations offrequency and mr where the normalized Q-factor is greater than or equalto 90%.

Comparison of FIG. 9 and FIG. 10 shows that a only one reflector elementis generally less effective that two reflector elements for at improvingnormalized Q-factor. However, in some applications one reflector elementat each end of an IDT may be sufficient. In this example, one reflectorelement (at each end of the IDT) with mr=0.75 microns provides asubstantial improvement in normalized Q-factor for a frequency range ofabout 4770 MHz to 4970 MHz.

FIG. 11 shows a chart 1100 illustrating the relationship between themark mr of reflector elements and frequency for an exemplary XBAR devicewith five reflector elements at each end of an IDT and pr=5.2 microns.The exemplary XBAR device is the same as the previous examples. As inFIG. 9, lighter shaded areas such as area 1110 identify combinations offrequency and mr where the normalized Q-factor is greater than or equalto 85%. Darker shaded areas such as area 1120 identify combinations offrequency and mr where the normalized Q-factor is greater than or equalto 90%.

Comparison of FIG. 9 and FIG. 11 shows that five reflector elements donot provide any significant improvement over two reflector elements.

FIG. 12 is a chart of the performance of an exemplary XBAR bandpassfilter with and without reflector elements. Specifically, the solid line1210 is a plot of the magnitude of S₂₁ (the input-output transferfunction) of a Band N77 filter with two reflector elements at each endof the XBARs in the filter. The reflector elements of shunt resonatorswere optimized for a frequency of 3.35 GHz and the reflector elements ofseries resonators were optimized for a frequency of 4.2 GHz. Thesefrequencies are at or near the edges of the N77 band where it istypically most difficult to achieve requirements on minimum S₂₁. Thedashed line 1220 is a plot of the magnitude of S₂₁ for the same filterwithout reflector elements on the XBARs. All of the data was developedby simulations of the filters using a finite element method.

The inclusion of reflector elements improves S₂₁ by 0.2 db at 3.35 GHzand by 0.4 dB at 4.2 GHz. Note, however, that the inclusion of thereflector elements reduces S₂₁ by as much as 0.25 dB at otherfrequencies, illustrating a tradeoff to be made during the design of anXBAR filter. In the exemplary bandpass filter of FIG. 12, the reflectorelements for all of the shunt resonators were selected for maximumQ-factor at the same frequency (3.35 GHz) and the reflector elements forall of the series resonators were selected for maximum Q-factor at thesame frequency (4.2 GHz). Further improvements in the filter transferfunction are likely if the reflector elements for each resonator areoptimized independently.

Description of Methods

FIG. 13 is a simplified flow chart summarizing a process 1300 forfabricating a filter device incorporating XBARs. Specifically, theprocess 1300 is for fabricating a filter device including multipleXBARs, some of which may include a frequency setting dielectric layer.The process 1300 starts at 1305 with a device substrate and a thin plateof piezoelectric material disposed on a sacrificial substrate. Theprocess 1300 ends at 1395 with a completed filter device. The flow chartof FIG. 13 includes only major process steps. Various conventionalprocess steps (e.g. surface preparation, cleaning, inspection, baking,annealing, monitoring, testing, etc.) may be performed before, between,after, and during the steps shown in FIG. 13.

While FIG. 13 generally describes a process for fabricating a singlefilter device, multiple filter devices may be fabricated simultaneouslyon a common wafer (consisting of a piezoelectric plate bonded to asubstrate). In this case, each step of the process 1300 may be performedconcurrently on all of the filter devices on the wafer.

The flow chart of FIG. 13 captures three variations of the process 1300for making an XBAR which differ in when and how cavities are formed inthe device substrate. The cavities may be formed at steps 1310A, 1310B,or 1310C. Only one of these steps is performed in each of the threevariations of the process 1300.

The piezoelectric plate may be, for example, lithium niobate or lithiumtantalate, either of which may be Z-cut, rotated Z-cut, or rotatedYX-cut. The piezoelectric plate may be some other material and/or someother cut. The device substrate may preferably be silicon. The devicesubstrate may be some other material that allows formation of deepcavities by etching or other processing.

In one variation of the process 1300, one or more cavities are formed inthe device substrate at 1310A, before the piezoelectric plate is bondedto the substrate at 1315. A separate cavity may be formed for eachresonator in a filter device. The one or more cavities may be formedusing conventional photolithographic and etching techniques. Typically,the cavities formed at 1310A will not penetrate through the devicesubstrate.

At 1315, the piezoelectric plate is bonded to the device substrate. Thepiezoelectric plate and the device substrate may be bonded by a waferbonding process. Typically, the mating surfaces of the device substrateand the piezoelectric plate are highly polished. One or more layers ofintermediate materials, such as an oxide or metal, may be formed ordeposited on the mating surface of one or both of the piezoelectricplate and the device substrate. One or both mating surfaces may beactivated using, for example, a plasma process. The mating surfaces maythen be pressed together with considerable force to establish molecularbonds between the piezoelectric plate and the device substrate orintermediate material layers.

At 1320, the sacrificial substrate may be removed. For example, thepiezoelectric plate and the sacrificial substrate may be a wafer ofpiezoelectric material that has been ion implanted to create defects inthe crystal structure along a plane that defines a boundary between whatwill become the piezoelectric plate and the sacrificial substrate. At1320, the wafer may be split along the defect plane, for example bythermal shock, detaching the sacrificial substrate and leaving thepiezoelectric plate bonded to the device substrate. The exposed surfaceof the piezoelectric plate may be polished or processed in some mannerafter the sacrificial substrate is detached.

Thin plates of single-crystal piezoelectric materials laminated to anon-piezoelectric substrate are commercially available. At the time ofthis application, both lithium niobate and lithium tantalate plates areavailable bonded to various substrates including silicon, quartz, andfused silica. Thin plates of other piezoelectric materials may beavailable now or in the future. The thickness of the piezoelectric platemay be between 300 nm and 1000 nm. When the substrate is silicon, alayer of SiO₂ may be disposed between the piezoelectric plate and thesubstrate. When a commercially available piezoelectric plate/devicesubstrate laminate is used, steps 1310A, 1315, and 1320 of the process1300 are not performed.

A first conductor pattern, including IDTs and reflector elements of eachXBAR, is formed at 1345 by depositing and patterning one or moreconductor layers on the front side of the piezoelectric plate. Theconductor layer may be, for example, aluminum, an aluminum alloy,copper, a copper alloy, or some other conductive metal. Optionally, oneor more layers of other materials may be disposed below (i.e. betweenthe conductor layer and the piezoelectric plate) and/or on top of theconductor layer. For example, a thin film of titanium, chrome, or othermetal may be used to improve the adhesion between the conductor layerand the piezoelectric plate. A second conductor pattern of gold,aluminum, copper or other higher conductivity metal may be formed overportions of the first conductor pattern (for example the IDT bus barsand interconnections between the IDTs).

Each conductor pattern may be formed at 1345 by depositing the conductorlayer and, optionally, one or more other metal layers in sequence overthe surface of the piezoelectric plate. The excess metal may then beremoved by etching through patterned photoresist. The conductor layercan be etched, for example, by plasma etching, reactive ion etching, wetchemical etching, or other etching techniques.

Alternatively, each conductor pattern may be formed at 1345 using alift-off process. Photoresist may be deposited over the piezoelectricplate. and patterned to define the conductor pattern. The conductorlayer and, optionally, one or more other layers may be deposited insequence over the surface of the piezoelectric plate. The photoresistmay then be removed, which removes the excess material, leaving theconductor pattern.

At 1350, one or more frequency setting dielectric layer(s) may be formedby depositing one or more layers of dielectric material on the frontside of the piezoelectric plate. For example, a dielectric layer may beformed over the shunt resonators to lower the frequencies of the shuntresonators relative to the frequencies of the series resonators. The oneor more dielectric layers may be deposited using a conventionaldeposition technique such as physical vapor deposition, atomic layerdeposition, chemical vapor deposition, or some other method. One or morelithography processes (using photomasks) may be used to limit thedeposition of the dielectric layers to selected areas of thepiezoelectric plate. For example, a mask may be used to limit adielectric layer to cover only the shunt resonators.

At 1355, a passivation/tuning dielectric layer is deposited over thepiezoelectric plate and conductor patterns. The passivation/tuningdielectric layer may cover the entire surface of the filter except forpads for electrical connections to circuitry external to the filter. Insome instantiations of the process 1300, the passivation/tuningdielectric layer may be formed after the cavities in the devicesubstrate are etched at either 1310B or 1310C.

In a second variation of the process 1300, one or more cavities areformed in the back side of the device substrate at 1310B. A separatecavity may be formed for each resonator in a filter device. The one ormore cavities may be formed using an anisotropic ororientation-dependent dry or wet etch to open holes through the backside of the device substrate to the piezoelectric plate. In this case,the resulting resonator devices will have a cross-section as shown inFIG. 1.

In a third variation of the process 1300, one or more cavities in theform of recesses in the device substrate may be formed at 1310C byetching the substrate using an etchant introduced through openings inthe piezoelectric plate. A separate cavity may be formed for eachresonator in a filter device. The one or more cavities formed at 1310Cwill not penetrate through the device substrate.

Ideally, after the cavities are formed at 1310B or 1310C, most or all ofthe filter devices on a wafer will meet a set of performancerequirements. However, normal process tolerances will result invariations in parameters such as the thicknesses of dielectric layerformed at 1350 and 1355, variations in the thickness and line widths ofconductors and IDT fingers formed at 1345, and variations in thethickness of the piezoelectric plate. These variations contribute todeviations of the filter device performance from the set of performancerequirements.

To improve the yield of filter devices meeting the performancerequirements, frequency tuning may be performed by selectively adjustingthe thickness of the passivation/tuning layer deposited over theresonators at 1355. The frequency of a filter device passband can belowered by adding material to the passivation/tuning layer, and thefrequency of the filter device passband can be increased by removingmaterial to the passivation/tuning layer. Typically, the process 1300 isbiased to produce filter devices with passbands that are initially lowerthan a required frequency range but can be tuned to the desiredfrequency range by removing material from the surface of thepassivation/tuning layer.

At 1360, a probe card or other means may be used to make electricalconnections with the filter to allow radio frequency (RF) tests andmeasurements of filter characteristics such as input-output transferfunction. Typically, RF measurements are made on all, or a largeportion, of the filter devices fabricated simultaneously on a commonpiezoelectric plate and substrate.

At 1365, global frequency tuning may be performed by removing materialfrom the surface of the passivation/tuning layer using a selectivematerial removal tool such as, for example, a scanning ion mill aspreviously described. “Global” tuning is performed with a spatialresolution equal to or larger than an individual filter device. Theobjective of global tuning is to move the passband of each filter devicetowards a desired frequency range. The test results from 1360 may beprocessed to generate a global contour map indicating the amount ofmaterial to be removed as a function of two-dimensional position on thewafer. The material is then removed in accordance with the contour mapusing the selective material removal tool.

At 1370, local frequency tuning may be performed in addition to, orinstead of, the global frequency tuning performed at 1365. “Local”frequency tuning is performed with a spatial resolution smaller than anindividual filter device. The test results from 1360 may be processed togenerate a map indicating the amount of material to be removed at eachfilter device. Local frequency tuning may require the use of a mask torestrict the size of the areas from which material is removed. Forexample, a first mask may be used to restrict tuning to only shuntresonators, and a second mask may be subsequently used to restricttuning to only series resonators (or vice versa). This would allowindependent tuning of the lower band edge (by tuning shunt resonators)and upper band edge (by tuning series resonators) of the filter devices.

After frequency tuning at 1365 and/or 1370, the filter device iscompleted at 1375. Actions that may occur at 1375 include formingbonding pads or solder bumps or other means for making connectionbetween the device and external circuitry (if such pads were not formedat 1345); excising individual filter devices from a wafer containingmultiple filter devices; other packaging steps; and additional testing.After each filter device is completed, the process ends at 1395.

Closing Comments

Throughout this description, the embodiments and examples shown shouldbe considered as exemplars, rather than limitations on the apparatus andprocedures disclosed or claimed. Although many of the examples presentedherein involve specific combinations of method acts or system elements,it should be understood that those acts and those elements may becombined in other ways to accomplish the same objectives. With regard toflowcharts, additional and fewer steps may be taken, and the steps asshown may be combined or further refined to achieve the methodsdescribed herein. Acts, elements and features discussed only inconnection with one embodiment are not intended to be excluded from asimilar role in other embodiments.

As used herein, “plurality” means two or more. As used herein, a “set”of items may include one or more of such items. As used herein, whetherin the written description or the claims, the terms “comprising”,“including”, “carrying”, “having”, “containing”, “involving”, and thelike are to be understood to be open-ended, i.e., to mean including butnot limited to. Only the transitional phrases “consisting of” and“consisting essentially of”, respectively, are closed or semi-closedtransitional phrases with respect to claims. Use of ordinal terms suchas “first”, “second”, “third”, etc., in the claims to modify a claimelement does not by itself connote any priority, precedence, or order ofone claim element over another or the temporal order in which acts of amethod are performed, but are used merely as labels to distinguish oneclaim element having a certain name from another element having a samename (but for use of the ordinal term) to distinguish the claimelements. As used herein, “and/or” means that the listed items arealternatives, but the alternatives also include any combination of thelisted items.

It is claimed:
 1. An acoustic resonator device comprising: apiezoelectric plate; and a conductor pattern formed on a surface of thepiezoelectric plate, the conductor pattern comprising: an interdigitaltransducer (IDT) including a first busbar, a second busbar, and ninterleaved parallel fingers, where n is a positive integer, wherein thefingers extend alternately from the first and second busbars, and thefingers include a first finger and an n'th finger at opposing ends ofthe IDT; a first reflector element proximate and parallel to the firstfinger; and a second reflector element proximate and parallel to then'th finger, wherein when an RF signal is applied between the first andsecond busbars, the first reflector element is at substantially the samepotential as the first finger and the second reflector element is atsubstantially the same potential as the n'th finger.
 2. The device ofclaim 1, wherein the first reflector element and the first finger areconnected to the same one of the first and second busbar, and a secondreflector element and the n'th finger are connected to the same one ofthe first and second busbar.
 3. The device of claim 1, wherein the firstreflector element is capacitively coupled to the first finger, and thesecond reflector element is capacitively coupled to the n'th finger. 4.The device of claim 1, wherein a distance pr between the first reflectorelement and the first finger and between the second reflector elementand the n'th finger is greater than or equal to 1.2 times a pitch p ofthe IDT.
 5. The device of claim 1, wherein a mark mr of the first andsecond reflector elements is configured to improve a Q-factor of thedevice at a predetermined frequency.
 6. The device of claim 1, whereinthe device is a shunt resonator in a ladder bandpass filter circuithaving a passband, and mr is selected to improve a Q-factor of thedevice at a lower edge of the passband.
 7. The device of claim 1,wherein the device is a series resonator in a ladder bandpass filtercircuit having a passband, and mr is selected to improve a Q-factor ofthe device at an upper edge of the passband.
 8. The device of claim 1,the conductor pattern further comprising: a third reflector elementproximate and parallel to the first reflector element, the thirdreflector element disposed such that the first reflector element iscentered between the third reflector element and the first finger; and afourth reflector element proximate and parallel to the second reflectorelement, the fourth reflector element disposed such that the secondreflector element is centered between the fourth reflector element andthe n'th finger.
 9. The device of claim 8, wherein the first and thirdreflector elements and the first finger are connected to the same one ofthe first and second busbar, and the second and fourth reflectorelements and the n'th finger are connected to the same one of the firstand second busbar.
 10. The device of claim 8, wherein the first andthird reflector elements are connected to each other and capacitivelycoupled to the first finger, and the second and fourth reflectorelements are connected to each other and capacitively coupled to then'th finger.
 11. The device of claim 8, wherein a distance pr betweenthe first reflector element and the first finger, between the first andthird reflector elements, between the second and fourth reflectorelements, and between the second reflector element and the n'th fingeris greater than or equal to 1.2 times a pitch p of the IDT.
 12. Thedevice of claim 8, wherein a mark mr of the first, second, third, andfourth reflector elements is configured to improve a Q-factor of thedevice at a predetermined frequency.
 13. The device of claim 12, whereinthe device is a shunt resonator in a ladder bandpass filter circuithaving a passband, and mr is selected to improve a Q-factor of thedevice at a lower edge of the passband.
 14. The device of claim 12,wherein the device is a series resonator in a ladder bandpass filtercircuit having a passband, and mr is selected to improve a Q-factor ofthe device at an upper edge of the passband.
 15. A bandpass filter,comprising: three or more acoustic resonators connected in a ladderfilter circuit, wherein each acoustic resonator comprises aninterdigital transducer (IDT) and one or two reflector elements at eachend of the IDT.
 16. The bandpass filter of claim 15, wherein, for eachof the three or more acoustic resonators: a respective pitch and arespective mark of the reflector elements are selected to improve aQ-factor of the acoustic resonator at a respective target frequency. 17.The bandpass filter of claim 15, wherein the three or more acousticresonator include at least one shunt resonator, and the pitch and markof the reflector elements of the shunt resonator are selected to improvea Q-factor of the shunt resonator at a lower edge of a passband of thebandpass filter.
 18. The bandpass filter of claim 15, wherein the threeor more acoustic resonator include at least one series resonator, andthe pitch and mark of the reflector elements of the series resonator areselected to improve a Q-factor of the shunt resonator at an upper edgeof a passband of the bandpass filter.
 19. An acoustic resonator devicecomprising: a substrate; a piezoelectric plate, a portion of thepiezoelectric plate forming a diaphragm spanning a cavity in thesubstrate; and a conductor pattern formed on a surface of thepiezoelectric plate, the conductor pattern comprising: an interdigitaltransducer (IDT) including a first busbar, a second busbar, and ninterleaved parallel fingers, where n is a positive integer, wherein thefingers extend alternately from the first and second busbars, and thefingers include a first finger and an n'th finger at opposing ends ofthe IDT; a first reflector element proximate and parallel to the firstfinger; and a second reflector element proximate and parallel to then'th finger, wherein the n interleaved parallel fingers, the firstreflector element, and the second reflector element are disposed on thediaphragm.
 20. The device of claim 17, wherein a distance pr between thefirst reflector element and the first finger and between the secondreflector element and the n'th finger is greater than or equal to 1.2times a pitch p of the IDT.
 21. The device of claim 17, wherein a markmr of the first and second reflector elements is configured to improve aQ-factor of the device at a predetermined frequency.
 22. The device ofclaim 17, the conductor pattern further comprising: a third reflectorelement proximate and parallel to the first reflector element, the thirdreflector element disposed such that the first reflector element iscentered between the third reflector element and the first finger; and afourth reflector element proximate and parallel to the second reflectorelement, the fourth reflector element disposed such that the secondreflector element is centered between the fourth reflector element andthe n'th finger, wherein the third and fourth reflector elements aredisposed on the diaphragm.